An electrographic image forming apparatus, such as a copying machine or a printer, includes a fixing device fixing a toner image onto a recording medium, such as a sheet. The general fixing device includes a fixing roller having an elastic layer formed around an outer circumference of a core metal, a pressuring roller pressure-contacting with this and a heating device arranged inside or outside the fixing roller. Such a fixing device is configured so as to fix a heated and melted toner onto the sheet by passing the sheet having the toner image transferred through a fixing nip formed between respective rotating rollers.
In the general fixing device, surface hardness of the fixing roller (the elastic layer) is set lower (softer) than surface hardness of the pressuring roller. In recent years, for purposes of acceleration of image forming, a pressing force of the pressuring roller is set high. It is because a distance of the fixing nip is lengthened by greatly deforming the soft fixing roller (the elastic layer). In this case, because the elastic layer is also greatly deformed in an axial direction, there was a problem that stress is concentrated on an interface between the core metal and elastic layer and the elastic layer is peeled from the core metal at both end parts in the axial direction.
In order to solve such a problem, as shown in FIG. 4, a fixing device 100 is proposed, which includes a fixing roller 101 having an endless belt 105 provided around a core metal 103 via an elastic layer 104 and a pressuring roller 102 pressure-contacting with this to form a nip part 106, and has stepped down parts G, in which parts contacting with the endless belt 105 are cut off at both end parts of the elastic layer 104 of the fixing roller 101 (refer to Patent Document 1). This fixing device is configured so as to restrain interfacial peeling of the elastic layer 104 from the core metal 103 because both end parts of the elastic layer 102 being the stepped down parts are not pressured by the pressuring roller 104.